1. Field of the Invention
The present invention relates to an ultrasonic diagnostic apparatus having a function of measuring intima media thickness (IMT) of a blood vessel based on reception signals obtained by transmitting ultrasonic waves to an object to be inspected and receiving ultrasonic waves from the object. Further, the present invention relates to an IMT measurement method and an IMF measurement program to be used when such IMT measurement is performed.
2. Description of a Related Art
Recent years, intima media thickness has attracted attention as an indicator for determination of arteriosclerosis. Referring to FIG. 9, an arterial wall has a three-layer structure including an intima 901, a media 902, and an adventitia 903. IMT refers to a thickness of the intima 901 and the media 902 of them, i.e., a length from a boundary between a vascular lumen 900 and the intima 901 to a boundary between the media 902 and the adventitia 903. From recent research, it has been found that the intima media thickness increases and a plaque is formed as arteriosclerosis progresses. Here, the plaque is a part where a vessel wall is raised inwardly. Tissues of the plaque may be various tissues such as blood clot or fatty or fibrous tissues, and cause carotid artery stenosis, cerebral infarction, cerebral ischemia, and so on.
FIGS. 10 and 11 are schematic diagrams showing part of a carotid artery. As shown in FIGS. 10 and 11, the blood pumped from the heart is introduced into a common carotid artery (CCA) 911, and divided into an external carotid artery (ECA) 912 that connects to an artery of the face and an internal carotid artery (ICA) 913 that connects to an artery of the brain.
The above-explained IMT is measured by ultrasonic examination (carotid artery ultrasonic examination). That is, an ultrasonic probe including an ultrasonic transducer array, in which plural ultrasonic transducers are arranged, is brought into contact with the cervical part of an object to be inspected (a patient) to transmit ultrasonic waves. Here, the reason the IMT is measured in the carotid artery is that the carotid artery is a favorite site of arteriosclerosis. In this regard, the plural ultrasonic transducers are sequentially driven and an ultrasonic beam is formed by synchronization of plural ultrasonic waves, and thereby, the object is electronically scanned. Thus transmitted ultrasonic waves are reflected on the surface of a structure within the object (a boundary between different tissues), and resulting ultrasonic echoes are received by the ultrasonic probe and reception signals are generated. Those reception signals are processed in an ultrasonic diagnostic apparatus main body connected to the ultrasonic probe, and thereby, an ultrasonic image is generated. Further, an examiner (an operator such as a doctor) measures the vessel wall by using a vernier caliper or the like in the ultrasonic image generated as described above, and therefore, the IMT is obtained. Furthermore, the level of arteriosclerosis is measured based on the IMT, and the vessel status throughout the body including the heart and the brain is estimated based on the result.
However, according to the measurement method, there are problems that the measurement requires long time and the measurement accuracy largely varies depending on the levels of skill of examiners. In order to actively utilize IMT in mass checkup or the like, efficient IMT measurement requiring short time and providing measurement results that vary little depending on examiners is desired.
As a related technology, Hiroyuki TOIDE, “PROPER ULTRASONIC EXAMINATION OF BLOOD VESSELS”, Ultrasonic examination technique, Vol. 31, No. 2 (2006), pp. 80 discloses precautions and points for ultrasonic examination of blood vessels. For example, in carotid artery ultrasonic examination, the vessel diameter, maxIMT and meanIMT are measured. Here, a plaque 915 is likely formed in a region where the direction of blood flow changes like in the vicinity of the entrance of common carotid artery or the vicinity of vascular bifurcation (BIF) 914. Accordingly, as shown in FIG. 10, the maxIMT is measured at two positions of the common carotid artery 911 and a region (BIF to ICA) from the bulbous part (vascular bifurcation (BIF) 914) to the internal carotid artery 913. Further, as shown in FIG. 11, meanIMT is obtained by measuring maxIMT and two IMTs at positions “a” and “c” on both sides at 1 cm from it, and calculating an average value of the three points as follows: {maxIMT+IMT(a)+IMT(c)}/3.
Japanese Patent Application Publication JP-A-11-318896 discloses an intima media thickness measurement apparatus including an ultrasonic unit that outputs data of images imaged by using ultrasonic waves as digital data, a data transmission unit that transmits the digital output of the ultrasonic unit by using optical coupling, and a data analysis unit that calculates the intima media thickness of a blood vessel based on the image data of the blood vessel transmitted by the data transmission unit, and the data analysis unit calculates a reference position based on a moving average value of intensity values of the digital image data and calculates the intima media thickness of the blood vessel based on the local maximum value and the local minimum value of the intensity values within a predetermined pixel range from the reference position toward the vessel wall of the blood vessel.
In JP-A-11-318896, the IMT value is automatically calculated by searching for peak values of intensity. Here, the intensity representing the intima side boundary (the boundary between the vascular lumen and the intima) is not so high and it may be difficult to detect the boundary, and accordingly, regression curve correction is performed in JP-A-11-318896. Alternatively, an average value of detected points may be used. However, according to the method, when there are many points that cannot be detected, the accuracy in IMT measurement is reduced and the true value is impossible to be obtained when maxIMT is calculated.
Alternatively, simply searching for the boundary based on the peak values of intensity may cause detection errors when the noise is great. For example, as shown in FIG. 12, in the case where plural relatively large peaks (1)-(3) are observed in intensity data, although the maximum peak (3) is actually the adventitia side boundary (the boundary between the media and the adventitia) of IMT and the adjacent peak (2) is the intima side boundary, the first peak (1) is detected as the intima side boundary and the next peak (2) is detected as the adventitia side boundary when the boundary search is started from the base position. Thus, diagnoses utilizing maxIMT will be largely affected.
Japanese Patent Application Publication JP-P2004-357892A discloses an ultrasonic diagnostic apparatus that displays an image within a body of an examinee based on reflected ultrasonic waves acquired from within the body by using an ultrasonic probe and performs measurement of intima media thickness of a blood vessel. The ultrasonic diagnostic apparatus includes (a) measurement region setting means for setting a region as an object to be measured within the displayed image, (b) vessel wall detecting means for detecting vessel wall existing in one or plural positions within the measurement region, and (c) IMT measurement means for performing measurement of intima media thickness with respect to each of the detected vessel walls. The vessel wall detecting means detects positions where intensity values of pixels suddenly change, and then, determines the position of the vessel wall based on the distance between adjacent intensity value sudden change positions. The IMT measurement means determines the intima media thickness with respect to each vessel wall based on the distance between adjacent intensity value sudden change positions that belong to the same vessel wall.
Here, in a measurement line that passes through one vessel wall, intensity value sudden change positions are observed at two positions of the vascular lumen side boundary and the adventitia side boundary of the intima media thickness. Accordingly, in JP-P2004-357892A, in the case where three or more intensity value sudden change positions are detected in a certain measurement line, whether or not those intensity value sudden change positions belong to the same vessel wall is determined based on the distances between the positions (paragraph 0016). However, since a vessel wall has continuity in the azimuth direction, it is not rational that the vessel wall status is determined based on the intensity change in the distance (depth) direction.